In bank note processing machines bank notes are checked with the aid of sensors for example as to their type with respect to currency or denomination, as to their authenticity, or their state. For this purpose the bank notes are conveyed by a transport system and thereby pass through a sensor path with one or a plurality of sensors. From the prior art (e.g. DE 32 42 789 C1) bank note processing machines are known the sensors of which each have two sensor components which are arranged on opposite sides of the sensor path. In the sensor components there are arranged sensitive measuring elements which must be protected from environmental influences—for example dust and soil particles which are brought into the machine by the bank notes.
With some sensors the bank notes guided past the sensor components touch the front side of the housing of the sensor component facing the bank notes. Here in the course of time the front side wears out which can impair the transport of the bank notes. For maintaining the bank note processing machines installed at the customer, in the case of wear complete sensor components must be replaced by new ones. After the replacement a readjustment of the replaced sensor components may also be necessary. Altogether, this means that a great effort is necessary and that costs for maintaining the sensors of bank note processing machines will accrue.
When bank notes are guided past sensors in a fashion according to the prior art, in case of high transport speeds a bank note jam may occur. Critical points are, in particular, narrow places and transitions between different components which laterally limit the transport path of the bank notes.
Therefore, it is the object of the invention to reduce the effort for maintaining the sensors and to achieve a trouble-free transport of the sheet material along the sensors even at high transport speeds.